Showing papers on "Earth's magnetic field published in 2003"

The extreme magnetic storm of 1–2 September 1859

Abstract: Using empirical results on the interplanetary magnetic field strengths of magnetic clouds versus velocities, we show that the 1 September 1859 Carrington solar flare most likely had an associated intense magnetic cloud ejection which led to a storm on Earth of DST ~ -1760 nT.

Resonant acceleration and diffusion of outer zone electrons in an asymmetric geomagnetic field

Abstract: [1] The outer zone radiation belt consists of energetic electrons drifting in closed orbits encircling the Earth between ∼3 and 7 RE Electron fluxes in the outer belt show a strong correlation with solar and magnetospheric activity, generally increasing during geomagnetic storms with associated high solar wind speeds, and increasing in the presence of magnetospheric ULF waves in the Pc-5 frequency range In this paper, we examine the influence of Pc-5 ULF waves on energetic electrons drifting in an asymmetric, compressed dipole and find that such particles may be efficiently accelerated through a drift-resonant interaction with the waves We find that the efficiency of this acceleration increases with increasing magnetospheric distortion (such as may be attributed to increased solar wind pressure associated with high solar wind speeds) and with increasing ULF wave activity A preponderance of ULF power in the dawn and dusk flanks is shown to be consistent with the proposed acceleration mechanism Under a continuum of wave modes and frequencies, we find that the drift resonant acceleration process leads to additional modes of radial diffusion in the outer belts, with timescales that may be appropriate to those observed during geomagnetic storms

Predicting interplanetary magnetic field (IMF) propagation delay times using the minimum variance technique

Abstract: [1] It has been known that the fluctuations in the interplanetary magnetic field (IMF) may be oriented in approximately planar structures that are tilted with respect to the solar wind propagation direction along the Sun-Earth line. This tilting causes the IMF propagating from a point of measurement to arrive at other locations with a timing that may be significantly different from what would be expected. The differences between expected and actual arrival times may exceed an hour, and the tilt angles and subsequent delays may have substantial changes in just a few minutes. A consequence of the tilting of phase planes is that predictions of the effects of the IMF at the Earth, on the basis of IMF measurements far upstream in the solar wind, will suffer from reduced accuracy in the timing of events. It has recently been shown how the tilt angles may be determined using multiple satellite measurements. However, since the multiple satellite technique cannot be used with real-time data from a single sentry satellite, then an alternative method is required to derive the phase front angles, which can then be used for more accurate predictions. In this paper we show that the minimum variance analysis (MVA) technique can be used to adequately determine the variable tilt of the plane of propagation. The number of points that is required to compute the variance matrix has been found to be much higher than expected, corresponding to a time period in the range of 7 to 30 min. The optimal parameters for the MVA were determined by a comparison of simultaneous IMF measurements from four satellites. With use of the optimized parameters it is shown that the MVA method performs reasonably well for predicting the actual time lags in the propagation between multiple spacecraft, as well as to the Earth. Application of this technique can correct for errors, on the order of 30 min or more, in the timing of predictions of geomagnetic effects on the ground.

Penetration of the solar wind electric field into the magnetosphere/ionosphere system

Abstract: [1] On April 17, 2002 an intense, long duration electric field penetration event was captured by the Jicamarca incoherent scatter radar. Other radars in the U. S. chain detected the event as well, although not with as much clarity. The Interplanetary Electric Field (IEF) is available from the ACE satellite as well. The ratio of the dawn-to-dusk component of the IEF to the dawn-to-dusk electric field in the equatorial ionosphere for periods less than about two hours is 15:1. We suggest that this corresponds to the ratio of the size of the magnetosphere to the length of the connection line between the Interplanetary Magnetic Field (IMF) and the Earth's magnetic field. Simultaneous magnetic field measurements at Piura (off the magnetic equator) and at Jicamarca (under the magnetic equator) in Peru, reveal the same high frequency components and suggest that a chain of stations or an equatorial fleet of satellites in low earth orbit could be used to monitor the connection length continuously.

Time variations in geomagnetic intensity

Abstract: [1] After many years spent by paleomagnetists studying the directional behavior of the Earth’s magnetic field at all possible timescales, detailed measurements of field intensity are now needed to document the variations of the entire vector and to analyze the time evolution of the field components. A significant step has been achieved by combining intensity records derived from archeological materials and from lava flows in order to extract the global field changes over the past 12 kyr. A second significant step was due to the emergence of coherent records of relative paleointensity using the remanent magnetization of sediments to retrace the evolution of the dipole field. A third step was the juxtaposition of these signals with those derived from cosmogenic isotopes. Contemporaneous with the acquisition of records, new techniques have been developed to constrain the geomagnetic origin of the signals. Much activity has also been devoted to improving the quality of determinations of absolute paleointensity from volcanic rocks with new materials, proper selection of samples, and investigations of complex changes in magnetization during laboratory experiments. Altogether these developments brought us from a situation where the field changes were restricted to the past 40 kyr to the emergence of a coherent picture of the changes in the geomagnetic dipole moment for at least the past 1 Myr. On longer timescales the field variability and its average behavior is relatively well documented for the past 400 Myr. Section 3 gives a summary of most methods and techniques that are presently used to track the field intensity changes in the past. In each case, current limits and potential promises are discussed. The section 4 describes the field variations measured so far over various timescales covered by the archeomagnetic and the paleomagnetic records. Preference has always been given to composite records and databases in order to extract and discuss major and global geomagnetic features. Special attention has been devoted to discussing the degree of confidence to be put in the data by considering the integration of multiple data sets involving different techniques and/or materials.

Observations of discrete, global magnetospheric oscillations directly driven by solar wind density variations

Abstract: [1] In this paper we present six events in which both the time series and the spectral content of solar wind number density fluctuations and magnetospheric magnetic field observations were highly correlated for intervals ranging from a few to twelve hours. The fluctuations were periodic and occurred at discrete frequencies which often matched the f = 1.3, 1.9, 2.6, and 3.4 mHz oscillations that have been attributed to global magnetospheric MHD cavity and/or waveguide modes. We also observed significant power in the sub-mHz region, with frequencies as low as f = 0.1 mHz. We show that these fluctuations were first observed in the solar wind, far upstream from the Earth, and argue that the convected density perturbations slowly alter the size of the magnetospheric cavity leading to the appearance of multiple, discrete magnetospheric oscillations. We argue that for these events the discrete frequencies were an inherent property of the solar wind and were not related to a possible cavity or waveguide mode of the magnetosphere. We then show that the density fluctuations, when converted into length scales, organize into scale sizes of L = 23, 30, 45, and 80–100 RE. Finally, we speculate on a possible solar source for the periodic solar wind structures.

Identification of Solar Sources of Major Geomagnetic Storms between 1996 and 2000

Abstract: This paper presents identification of solar coronal mass ejection (CME) sources for 27 major geomagnetic storms (defined by disturbance storm time index ≤ -100 nT) occurring between 1996 and 2000. Observations of CMEs and their solar surface origins are obtained from the Large Angle and Spectrometric Coronagraph (LASCO) and the EUV Imaging Telescope (EIT) instruments on the SOHO spacecraft. Our identification has two steps. The first step is to select candidate front-side halo (FSH) CMEs using a fixed 120 hr time window. The second step is to use solar wind data to provide further constraints, e.g., an adaptive time window defined based on the solar wind speed of the corresponding interplanetary CMEs. We finally find that 16 of the 27 (59%) major geomagnetic storms are identified with unique FSH CMEs. Six of the 27 events (22%) are associated with multiple FSH CMEs. These six events show complex solar wind flows and complex geomagnetic activity, which are probably the result of multiple halo CMEs interacting in interplanetary space. A complex event occurs when multiple FSH CMEs are produced within a short period. Four of the 27 (15%) events are associated with partial-halo gradual CMEs emerging from the east limb. The surface origin of these events is not known because of a lack of any EIT signature. We believe that they are longitudinally extended CMEs having a component moving along the Sun-Earth connection line. One of the 27 major geomagnetic storms is caused by a corotating interaction region. We find an asymmetry in the longitudinal distribution of solar source region for the CMEs responsible for major geomagnetic storms. They are more likely to originate from the western hemisphere than from the eastern hemisphere. In terms of latitude, most geoeffective CMEs originate within a latitude strip of ±30°. The average transit time for a solar CME to arrive at the near-Earth space is found to be 64 hr, while it takes 78 hr on average to reach the peak of the geomagnetic storm. There is a correlation between CME transit time from the Sun to the near-Earth space (T, in hours) and the CME initial velocity (V, in unit of kilometers per second) at the Sun, which can be simply described as T = 96 - (V/21). We also find that while these geoeffective CMEs are either full-halo CMEs (67%) or partial-halo CMEs (30%), there is no preference for them to be fast CMEs or to be associated with major flares and erupting filaments.

Storm‐time distortion of the inner magnetosphere: How severe can it get?

Abstract: [1] First results are presented of an effort to model the storm-time distortion of the magnetic field in the inner magnetosphere using space magnetometer data. Strong geomagnetic storms are relatively rare events, represented by only a small fraction of the data used in the derivation of existing empirical geomagnetic field models. Hence using those models for the mapping of the storm-time magnetosphere is at most an extrapolation based on trends, obtained from quiet and moderately disturbed data. To overcome that limitation, a set of data was created, containing only clear-cut events with Dst ≤ −65 nT, with the goal to derive models of the inner and near geomagnetic field (R < 15 RE), representing strongly disturbed geomagnetic configurations and their evolution during the storm cycle. The final data set included about 143,000 records with 5-min average B-vectors, covering 37 major storms between 1996 and 2000. Most of the data came from GOES-8, -9, -10, Polar, and Geotail spacecraft, and two storms in February–March of 1998 were also partially covered by the data of Equator-S. In all cases, only those storms were selected for which concurrent solar wind and IMF data were available for the entire duration of the event. Interplanetary medium data were provided by Wind, ACE, and, to a lesser extent, by IMP 8 and Geotail. The inner magnetospheric field was represented using the newly developed T01 model [Tsyganenko, 2002a, 2002b], with a duskside partial ring current with variable amplitude and scale size, an essential part of the storm-time current system. The modeling revealed an enormous distortion and dawn-dusk asymmetry of the inner magnetosphere during the peak of the storm main phase, caused by the combined effect of the symmetric and partial ring currents, cross-tail current, and Birkeland currents. We found that during storms with Dst < −250 nT the tail-like deformation of the nightside field penetrates so close to Earth that the quasi-dipolar approximation breaks down at distances as small as 3–4 RE. This finding yields a quantitative answer to the question of why the auroras expand to unusually low latitudes during extremely strong storms. It also may provide a natural explanation for the observed impulsive injections and energizations of charged particles on the innermost L-shells. Finally, it questions the validity of using the dipole or quasi-dipole approximation in numerical simulations of severe storms in the inner magnetosphere.

Tail plasma sheet models derived from Geotail particle data

Abstract: [1] Simple analytical models have been derived for the first time, describing the 2-D distribution (along and across the Earth's magnetotail) of the central plasma sheet (CPS) ion temperature, density, and pressure, as functions of the incoming solar wind and interplanetary magnetic field (IMF) parameters, at distances between 10 and 50 RE. The models are based on a large set of data of the Low-Energy Particle (LEP) and Magnetic Field (MGF) instruments, taken by Geotail spacecraft between 1994 and 1998, comprising 7234 1-min average values of the CPS temperature and density. Concurrent solar wind and IMF data were provided by the Wind and IMP 8 spacecraft. The accuracy of the models was gauged by the correlation coefficient (c.c.) R between the observed and predicted values of a parameter. The CPS ion density N is controlled mostly by the solar wind proton density and by the northward component of the IMF. Being the least stable characteristic of the CPS, it yielded the lowest c.c. RN = 0.57. The CPS temperature T, controlled mainly by the solar wind speed V and the IMF Bz, gave a higher c.c. RT = 0.71. The CPS ion pressure P was best controlled by the solar wind ram pressure Psw and by an IMF-related parameter F = B⟂, where B⟂ is the perpendicular component of the IMF and θ is its clock angle. In a striking contrast with N and T, the model pressure P revealed a very high c.c. with the data, RP = 0.95, an apparent consequence of the force balance between the CPS and the tail lobe magnetic field. No significant dawn-dusk asymmetry of the CPS was found beyond the distance 10 RE, in line with the observed symmetry of the tail lobe magnetic field. The plasma density N is lowest at midnight and increases toward the tail's flanks. Larger (smaller) solar wind ion densities and northward (southward) IMF Bz result in larger (smaller) N in the CPS. In contrast to the density N, the temperature T peaks at the midnight meridian and falls off toward the dawn/dusk flanks. Faster (slower) solar wind flow and southward (northward) IMF Bz result in a hotter (cooler) CPS. The CPS ion pressure P is essentially a function of only XGSM in the midtail (20–50 RE); at closer distances the isobars gradually bend to approximately follow the contours of constant geomagnetic field strength. For northward IMF conditions combined with a slow solar wind, the isobars remain quasi-circular up to larger distances, reflecting a weaker tail current and, hence, more dipole-like magnetic field.

First champ mission results for gravity, magnetic and atmospheric studies

Abstract: I Orbit and Earth Gravity Field.- CHAMP Orbit and Gravity Instrument Status.- On Board Evaluation of the STAR Accelerometer.- Determination of CHAMP Accelerometer Calibration Parameters.- CHAMP Accelerometer and Star Sensor Data Combination.- CHAMP Clock Error Characterization.- Determination of the CHAMP GPS Antenna with Respect to Satellite's Mass Center.- Spaceborne GPS for POD and Earth Science.- The CHAMP Orbit Comparison Campaign.- CHAMP Orbit Determination with GPS Phase-Connected, Precise Point Positioning.- Kinematic and Dynamic Determination of Trajectories for Low Earth Satellites Using GPS.- CHAMP Double-Difference Kinematic POD with Ambiguity Resolution.- Approaches to CHAMP Precise Orbit Determination.- STAR Accelerometer Contribution to Dynamic Orbit and Gravity Field Model Adjustment.- Impact of Different Data Combinations on the CHAMP Orbit Determination.- CHAMP Rapid Science Orbit Determination - Status and Future Prospects.- Orbit Predictions for CHAMP - Development and Status.- Thermospheric Events in CHAMP Precise Orbit Determination.- New Global Gravity Field Models from Selected CHAMP Data Sets.- First Insight into Temporal Gravity Variablility from CHAMP.- CHAMP Gravity Field Recovery with the Energy Balance Approach.- Preliminary Analysis of CHAMP State Vector and Accelerometer Data for the Recovery of the Gravity Potential.- CHAMP Precise Orbit Determination and Gravity Field Recovery.- Gravitational Field Modelling from CHAMP-Ephemerides by Harmonic Splines and Fast Multipole Techniques.- Evaluation of Geoid Models with GPS/Levelling Points in Sweden and Finland.- Geophysical Impact of Field Variations.- CHAMP, Mass Displacements and the Earth's Rotation.- CHAMP Gravity Anomalies over Antarctica.- Assimilation of Altimeter and Geoid Data into a Global Ocean Model.- Total Density Retrieval with STAR.- II Earth Magnetic Field.- CHAMP ME Data Processing and Open Issues.- Ion Drift-Meter Status and Calibration.- CO2 - A CHAMP Magnetic Field Model.- Decadal and Subdecadal Secular Variation of Main Geomagnetic Field.- Modelling the Earth's Magnetic Field: Wavelet Based and Standard Methods.- Improved Parameterization of External Magnetic Fields from CHAMP Measurements.- Monitoring Magnetospheric Contributions using Ground-Based and Satellite Magnetic Data.- Unraveling the Magnetic Mystery of the Earth's Lithosphere: The Background and the Role of the CHAMP Mission.- A Comparison of Global Lithospheric Field Models Derived from Satellite Magnetic Data.- Mapping the Lithospheric Magnetic Field from CHAMP Scalar and Vector Magnetic Data.- Improving the Definition of Cratonic Boundaries Utilizing the Lithospheric Magnetic Field derived from CHAMP Observations.- Crustal Magnetisation Distribution Deduced from CHAMP Data.- Multiscale Downward Continuation of CHAMP FGM-Data for Crustal Field Modelling.- CHAMP Enhances Utility of Satellite Magnetic Observations to Augment Near-Surface Magnetic Survey Coverage.- Comparing Magsat, Orsted and CHAMP Crustal Magnetic Anomaly Data over the Kursk Magnetic Anomaly, Russia.- CHAMP, Orsted and Magsat Magnetic Anomalies of the Antarctic Lithosphere.- Separation of External Magnetic Signal for Induction Studies.- Two-Dimensional Spatiotemporal Modelling of Satellite Electromagnetic Induction Signals.- Night-Time Ionospheric Currents.- Multiscale Determination of Radial Current Distribution from CHAMP FGM-Data.- Ionospheric Currents from CHAMP Magnetic Field Data - Comparison with Ground Based Measurements.- Mapping of Field-Aligned Current Patterns during Northward IMF.- Field-Aligned Currents Inferred from Low-Altitude Earth-Orbiting Satellites and Ionospheric Currents Inferred from Ground-Based Magnetometers - Do They Render Consistent Results?.- III Neutral Atmosphere and Ionosphere.- GPS Radio Occultation with CHAMP.- Validation and Data Quality of CHAMP Radio Occultation Data.- Global Climate Monitoring based on CHAMP/GPS Radio Occultation Data.- Initial Results on Ionosphere/Plasmasphere Sounding based on GPS Data Obtained On Board CHAMP.- Backpropagation Processing of GPS Radio Occultation Data.- Combination of NOAA16/ATOVS Brightness Temperatures and the CHAMP Data to get Temperature and Humidity Profiles.- An Improvement of Retrieval Techniques for Ionospheric Radio Occultations.- Validation of Water Vapour Profiles from GPS Radio Occultations in the Arctic.- Comparison of DMI-Retrieval of CHAMP Occultation Data with ECMWF.- The Assimilation of Radio Occultation Measurements.- Status of Ionospheric Radio Occultation CHAMP Data Analysis and Validation of Higher Level Data Products.- NWP Model Specific Humidities Compared with CHAMP/GPS and TERRA/MODIS Data.- Analysis of Gravity Waves from Radio Occultation Measurements.- GPS Atmosphere and Ionosphere Methods used on Orsted Data and Initial Application on CHAMP Data.- Combining Radio Occultation Measurements with Other Instruments to Map the Ionospheric Electron Concentration.- Vertical Gradients of Refractivity in the Mesosphere and Atmosphere Retrieved from GPS/MET and CHAMP Radio Occultation Data.- Observation of Reflected Signals in MIR/GEO and GPS/MET Radio Occultation Missions.- Assimilation Experiments of One-dimensional Variational Analyses with GPS/MET Refractivity.- Monitoring the 3 Dimensional Ionospheric Electron Distribution based on GPS Measurements.- Comparison of Three Different Meteorological Datasets (ECMWF, Met Office and NCEP).- Radio Occultation Data Processing at the COSMIC Data Analysis and Archival Center (CDAAC).- Verification of CHAMP Radio-Occultation Observations in the Ionosphere Using MIDAS.- Approach to the Cross-Validation of MIPAS and CHAMP Temperature and Water Vapour Profiles.- Author Index.- Keyword Index.

Martian magnetic morphology: Contributions from the solar wind and crust

Abstract: [1] Mars Global Surveyor (MGS) Magnetometer (MAG) data provide constraints on magnetic morphology at Mars, including the relative importance of the solar wind and of crustal magnetic sources. We analyze MAG data to characterize the upstream interplanetary magnetic field (IMF) and confirm trends in the magnetic field expected from the solar wind interaction with a planetary atmosphere, including increases at the shock and magnetic pile-up boundary (MPB), postshock turbulence, and field line draping around the Martian obstacle. Crustal magnetic sources locally modify the solar wind interaction, adding variability to the Martian magnetic environment that depends on planetary rotation. We identify trends in the vector magnetic field with respect to altitude, solar zenith angle, and planetary location. Crustal sources influence the magnetic field to different altitudes above different regions, and the influence of the strongest source extends to 1300–1400 km. The draped IMF partially controls the field topology above crustal sources, and crustal magnetic field lines reconnect to this field in a systematic fashion that depends upon Mars' geography, IMF strength, and IMF orientation.

Storm sudden commencement events and the associated geomagnetically induced current risks to ground-based systems at low-latitude and midlatitude locations

Abstract: [1] Large impulsive geomagnetic field disturbances from auroral current systems have always been well understood as a concern for power grids in close proximity to these disturbance regions, predominantly at high-latitude locations. Magnetospheric shocks (SSCs) due to large-scale interplanetary pressure pulses are familiar from a geomagnetic disturbance perspective but have not been understood in the context as a potential driver for large geomagnetically induced currents (GICs). Observational evidence and analysis contained in this paper illustrate such events are capable of producing large geoelectric fields and associated GIC risks at any latitude, even equatorial locations. A large SSC disturbance on 24 March 1991 produced some of the largest GICs ever measured in the United States at midlatitude locations. The analysis methods and understanding of electromagnetic coupling processes at that time were unable to fully explain these observations. Electrojet-driven disturbances common at high-latitude locations during geomagnetic substorms cause large amplitude variations in locally observed B field, while SSC events are characterized as low-amplitude B field disturbance events. Disturbance amplitude only accounts for part of the electromagnetic coupling process. The attribute of spectral content of the disturbance is equally important and heretofore had not been well understood and was not well measured unless high-cadence observations were conducted. The deep-earth ground conductivity also provides an important enabling role at higher frequencies. Deep-earth ground response to geomagnetic field disturbances is highly frequency-dependent. Therefore for nearly all ground conditions the higher the spectral content of the incident magnetic field disturbance, the higher the relative geoelectric field response.

Global response of the plasmasphere to a geomagnetic disturbance

Abstract: [1] Global images of the plasmasphere obtained by the Extreme Ultraviolet (EUV) imager on the IMAGE satellite are used to study the evolving structure of the plasmasphere during two geomagnetic disturbances. By tracking the location of the plasmapause as a function of L shell and magnetic local time, quantitative measurements of radial and azimuthal motions of the boundary are made for intervals ≥7 hours in duration with a time resolution of 10 min. The two cases presented are 26–27 June 2001, a relatively weak but isolated geomagnetic disturbance, and 9–10 June 2001, a moderate event with a multistaged onset and recurring substorm activity after the main disturbance. In both cases the onset of the disturbance, correlated with a southward turning of the IMF, is characterized by inward motion or erosion of the plasmapause and a smoothing of any existing azimuthal variations across the nightside. Over a period of many hours, a plasmaspheric plume forms in the afternoon sector as a result of sunward flows from dusk and corotational flows across the dayside. Azimuthal variations in the plasmapause radius tend to form in the local time sector from dawn to the western edge of the plume, including mesoscale (≤0.5 in L and ≤2 hours in local time) crenulations and larger-scale shoulder features, while the nightside boundary remains featureless. In the 26–27 June 2001 case, the magnetosphere entered a period of deep quiet after the main disturbance, and the plasmaspheric plume began to corotate with the main plasmasphere from the afternoon sector across the nightside. In contrast, the plume in the 9–10 June 2001 event became wrapped around the main plasmasphere and a second plume formed in the afternoon sector, perhaps due to continued substorm activity. In situ density data for these events show highly irregular density structure within the plumes as measured at geosynchronous orbit, whereas a measurement by IMAGE RPI suggests that there may be less structure near the base of the plume closer to the main plasmasphere.

Akebono/Suprathermal Mass Spectrometer observations of low‐energy ion outflow: Dependence on magnetic activity and solar wind conditions

Abstract: [1] We present observations by the Suprathermal Mass Spectrometer (SMS) on Akebono (EXOS-D) of ion outflow in the energy range from <1 to ∼70 eV. These observations cover a unique region of phase space and present an opportunity to “tie together” observations from disparate satellites. Variation of the total hemispheric O+ and H+ outflow rates with solar radio flux (monitored by the Penticton F10.7 index), with geomagnetic activity (monitored by the Kp index), and with solar wind parameters is discussed. Comparisons of F10.7 and Kp trends to results from Polar and Dynamics Explorer-1 (DE-1) lead us to conclude that flows of H+ in this low energy range are entirely sufficient to account for higher-energy flows at higher altitudes. On the other hand, we infer a substantial amount of O+ at energies above 70 eV. Both H+ and O+ outflow rates in this range exhibit a strong correlation with the solar wind kinetic pressure, the solar wind electric field, and the variability in the interplanetary magnetic field (IMF) in the hour preceding. While these factors are also associated with increased geomagnetic activity (Kp), a separate, Kp-independent effect is also found, showing a correlation of ion outflow with solar wind density and an anticorrelation with solar wind velocity.

Satellite observations of magnetic fields due to ocean tidal flow.

Abstract: The ocean is an electrically conducting fluid that generates secondary magnetic fields as it flows through Earth's main magnetic field. Extracting ocean flow signals from remote observations has become possible with the current generation of satellites measuring Earth's magnetic field. Here, we consider the magnetic fields generated by the ocean lunar semidiurnal (M2) tide and demonstrate that magnetic fields of oceanic origin can be clearly identified in satellite observations.

On the possible occurrence of "archaeomagnetic jerks" in the geomagnetic field over the last three millennia

Abstract: Abstract Archaeomagnetism can provide a high-resolution full-vector description of the Earth’s magnetic field for the past several thousand years. We analyse the bulk of archaeomagnetic data (both direction and intensity) obtained recently in Western Europe and the Eastern Mediterranean covering the past three millennia. We demonstrate a remarkable coincidence between sharp cusps in geomagnetic field direction and intensity maxima (two clear ones at ∼AD 200 and 1400; two presently less well constrained at ∼800 BC and AD 800). These sharp changes may constitute a new feature of geomagnetic secular variation (‘archaeomagnetic jerks’) with time characteristics intermediate between ‘geomagnetic jerks’ and ‘magnetic excursions’.

Ionospheric equivalent current distributions determined with the method of spherical elementary current systems

Abstract: [1] The ground magnetic field disturbance caused by ionospheric currents can be represented by equivalent currents placed to the ionospheric plane. Equivalent currents provide valuable information about the ionospheric electrodynamics, and thus they can be used, for example, in studies of space weather, ionosphere-magnetosphere coupling, and the magnetotelluric source effect. We derive equivalent currents by using the spherical elementary current system method. The applicability of the method for the Baltic Electromagnetic Array Research (BEAR) magnetometer array is validated by means of synthetic ionospheric current models and by investigating the goodness of the fit between the modeled and measured ground magnetic field. The applicability of the method for the sparser International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network is also proved. In addition, the combination of the elementary current system method and the complex image method, used for the calculation of the induced electromagnetic fields on ground, is introduced, and the combination of the methods is tested by using geoelectric field data from the BEAR project. Our special interest is in the effects that rapidly varying ionospheric currents have on technological conductor systems at the surface of the Earth due to geomagnetically induced currents. Comparison between equivalent currents and the time derivative vector of the horizontal magnetic field emphasizes the importance of small-scale structures.

Production of cosmogenic Be nuclei in the Earth's atmosphere by cosmic rays: Its dependence on solar modulation and the interstellar cosmic ray spectrum

Abstract: Recent work by McCracken [2001] shows that 10 Be production rates by cosmic rays on the polar plateau are little affected by geomagnetic field changes in the last few hundred years. Also, the 10 Be observed in ice cores on the polar plateau probably originated at high latitudes and precipitated to the Earth in about 1 year, according to McCracken. As a result of this assumption, ice core records of 10 Be concentration extending back several hundred years, including the Maunder minimum, have the potential to study the solar modulation of cosmic rays on a time scale extending back several hundred years. These ice core records indicate that the 10 Be concentration at the time of the Maunder minimum was ∼2.0 times what it was during recent sunspot minima in 1965 and 1976. We have examined 10 Be production in the atmosphere using new data related to the interstellar cosmic ray spectrum and the effects of solar modulation as determined from Voyager spacecraft data in the outer heliosphere. We have used the FLUKA Monte Carlo program along with new cross-section data to calculate the production of nucleons and 10 Be nuclei in the atmosphere. These calculations show that 10 Be temporal variations are sensitive indicators of low-energy solar modulation. Our calculations of 10 Be production are able to reproduce well the factor ∼1.5-2.0 change in 10 Be observed in the ice core data as a result of the 11-year solar modulation. We are also able to show that starting as recently as the sunspot minimum of 1954, the cosmic ray intensity at the Earth was higher than it was during more recent minima. The cosmic ray intensity during these minima time periods represents the residual modulation between the Earth and interstellar space. The 10 Be measurements are consistant with the fact that given the interstellar cosmic ray spectrum used in this analysis, this residual modulation was small or zero at the time of the Maunder minimum.

Effects on the ionosphere due to phenomena occurring below it

Abstract: The terrestrial thermosphere and ionosphere form the most variable part of theEarth's atmosphere. Because our society depends on technological systems thatcan be affected by thermospheric and ionospheric phenomena, understanding,monitoring and ultimately forecasting the changes of the thermosphere–ionosphere system are of crucial importance to communications, navigation and the exploration of near-Earth space. The reason for the extreme variability of the thermosphere–ionosphere system isits rapid response to external forcing from various sources, i.e., thesolar ionizing flux, energetic charged particles and electric fields imposed via the interaction between the solar wind, magnetosphere and ionosphere, as well as coupling from below (“meteorological influences”) by the upward propagating, broad spectrum,internal atmospheric waves (planetary waves, tides, gravity waves) generated in thestratosphere and troposphere. Thunderstorms, typhoons, hurricanes, tornadoes andeven seismological events may also have observable consequences in the ionosphere.The release of trace gases due to human activity have the potential to cause changes inthe lower and the upper atmosphere.A brief overview is presented concerning the discoveries and experimentalresults that have confirmed that the ionosphere is subject to meteorologicalcontrol (especially for geomagnetic quiet conditions and for middle latitudes).D-region aeronomy, the winter anomaly of radiowave absorption, wave-liketravelling ionospheric disturbances, the non-zonality and regional peculiaritiesof lower thermospheric winds, sporadic-E occurrence and structure, spread-Fevents, the variability of ionospheric electron density profiles and Total ElectronContent, the variability of foF2, etc., should all be considered in connection withtropospheric and stratospheric processes. “Ionospheric weather”, as a part of spaceweather, (i.e., hour-to-hour and day-to-day variability of the ionospheric parameters)awaits explanation and prediction within the framework of the climatological, seasonal,and solar-cycle variations.

ULF wave identification in the magnetosheath: The k-filtering technique applied to Cluster II data

Abstract: The magnetic fluctuations in the magnetosheath are studied, thanks to Cluster II data. The k-filtering technique is applied to explore ULF magnetic fluctuations using STAFF (Spatio-Temporal Analysis of a Field Fluctuations) data. Based on multipoint measurements, the k-filtering technique allows, for the first time, to estimate the Magnetic Field Energy Distribution (MFED) in both the angular frequency and wave vector space. We show how the localisation of the magnetic energy in the (ω, k) domain can be used to identify the linear modes that can propagate in the magnetosheath. A comparison between k-filtering results and prediction of the linear theory is performed. For the frequencies examined the magnetic energy seems to be distributed over the low frequency modes: mirror, Alfven, and slow modes. Estimation of Doppler shift shows that each frequency observed is the superposition of different frequencies in the plasma frame. This ``mixture of modes'' at a given observed frequency explains why the fluctuations are generally not observed to be polarized, as shown in previous studies. Some other implications on a weak turbulence approach of the magnetic fluctuations in the magnetosheath are discussed.

Influence of the solar wind dynamic pressure on the decay and injection of the ring current

Abstract: The influence of the solar wind dynamic pressure on the decay and injection of thering current is investigated empirically, on the basis of the solar wind and the geomagneticindex Dst of the OMNI database, for the period from January 1964 to July 2001. Wefound that when the position of the ring current is closer to the Earth for a higher solarwind dynamic pressure, the decay time of the ring current decreases. The decay time, inhours, varies as follows, t = 8.70 exp(6.66/(6.04 + P)), for northward interplanetarymagnetic fields (IMF), where P is the solar wind dynamic pressure in nanopascals. It isalso found, by minimizing the root mean square errors of the hourly Dst differencebetween the calculated values and the measured ones, that the ring current injection rate isproportional to the solar wind dynamic pressure, with a power index equal to 0.2 duringsouthward IMF. This implies that the ring current injection increases when themagnetosphere is more compressed by high solar wind dynamic pressure. On the basis ofour new results we demonstrate that the predictions of Dst using O’Brien andMcPherron’s [2000a] model are improved, especially for intense geomagnetic stormswhen the influence of the solar wind dynamic pressure on the decay and injection of ringcurrent is taken into consideration.

The diamagnetic effect of the equatorial Appleton anomaly: Its characteristics and impact on geomagnetic field modeling

Abstract: [1] The diamagnetic effect generally reduces the magnetic field inside a plasma. Its importance is appreciated in regions like the magnetosphere and the solar wind. In the ionosphere, depletions of the geomagnetic field have up to now been considered negligible. The CHAMP satellite provides for the first time the combination of high-resolution magnetic field measurements and plasma density observations on the same spacecraft in low-Earth orbit. We show the typical distribution of electron density at the altitude of about 430 km for various local times. Particularly prominent features are the density enhancements north and south of the dip equator. As expected, the magnetic field intensity is depressed in the crest region by an amount of more than 5 nT. The diamagnetic effect is strongest from sunset to midnight and thus causes errors in global geomagnetic field models which are usually computed from data sampled at all night-time hours.

The 9th-Generation International Geomagnetic Reference Field

Abstract: SUMMARY The International Association of Geomagnetism and Aeronomy has recently released the 9th-Generation International Geomagnetic Reference Field—the latest version of a standard mathematical description of the Earth's main magnetic field used widely in studies of the Earth's deep interior, its crust and its ionosphere and magnetosphere. The coefficients were recently finalized at the XXIII General Assembly of the International Union of Geophysics and Geodesy, held at Sapporo in Japan in 2003 July. The IGRF is the product of a huge collaborative effort between magnetic field modellers and the institutes involved in collecting and disseminating magnetic field data from satellites and from observatories and surveys around the world.

A semi-global reference model for electrical conductivity in the mid-mantle beneath the north Pacific region

Abstract: [1] One-dimensional electrical conductivity structure in the mid-mantle of the one-fourth of the Earth beneath the north Pacific Ocean was obtained by a semi-global electromagnetic induction study. Electromagnetic response functions estimated from electric field variations measured by submarine cables and geomagnetic field variations obtained by magnetic observatories and long-term observations sites were inverted into radially symmetric conductivity distribution by taking the distribution of land and ocean at the surface into account. As a most preferred model, a smooth conductivity-depth profile was obtained with two abrupt increases that possibly correspond to the seismic discontinuities at 410 and 660 km.

Analyses on the geometrical structure of magnetic field in the current sheet based on cluster measurements

Abstract: The geometrical structure of the magnetic field is a critical character in the magnetospheric dynamics. Using the magnetic field data measured by the Cluster constellation satellites, the geometrical structure including the curvature radius, directions of curvature, and normal of the osculating planes of the magnetic field lines within the current sheet/neutral sheet have been investigated. The results are ( 1) Inside of the tail neutral sheet (NS), the curvature of magnetic field lines points towards Earth, the normal of the osculating plane points duskward, and the characteristic half width ( or the minimum curvature radius) of the neutral sheet is generally less than 2 R-E, for many cases less than 1600 km. (2) Outside of the neutral sheet, the curvature of magnetic field lines pointed northward ( southward) at the north ( south) side of NS, the normal of the osculating plane points dawnward, and the curvature radius is about 5 R-E similar to 10 R-E. (3) Thin NS, where the magnetic field lines have the minimum of the curvature radius less than 0.25 R-E, may appear at all the local time between LT 20 hours and 4 hours, but thin NS occurs more frequently near to midnight than that at the dawnside and duskside. (4) The size of the NS is dependent on substorm phases. Generally, the NS is thin during the growth and expansion phases and grows thick during the recovery phase. (5) For the one-dimensional NS, the half thickness and flapping velocity of the NS could be quantitatively determined. Therefore the differential geometry analyses based on Cluster 4-point magnetic measurements open a window for visioning the three-dimensional static and dynamic magnetic field structure of geomagnetosphere.

A numerical simulation of the geomagnetic sudden commencement: 1. Generation of the field-aligned current associated with the preliminary impulse

Abstract: [1] The magnetospheric response to a solar wind impulse, a geomagnetic sudden commencement, is studied using an MHD model of the coupled solar wind-magnetosphere-ionosphere system. This paper discusses propagation of the first signal launched by the impulse and generation of the field-aligned current that causes the ground magnetic signal detected as the preliminary impulse (PI). It is revealed that the PI current is first excited as an enhanced Chapman-Ferraro current in the magnetopause and next turns to the magnetosphere along the wavefront of the compressional signal launched by the impulse. It is finally converted to a field-aligned current via mode coupling due to plasma nonuniformity. The current in the wavefront region is an inertia current. We present a quantitative model of the PI model presented by Araki [1994] by using a numerical simulation.